refactor(split): PEP 3102 keyword-only arguments

Author: tony

src/libtmux/pane.py

@@ -489,6 +489,7 @@ def select_pane(self) -> "Pane":
 
     def split(
         self,
+        /,
         start_directory: t.Optional[str] = None,
         attach: bool = False,
         direction: t.Optional[PaneDirection] = None,

src/libtmux/window.py

@@ -199,6 +199,7 @@ def select_pane(self, target_pane: t.Union[str, int]) -> t.Optional["Pane"]:
 
     def split(
         self,
+        /,
         target: t.Optional[t.Union[int, str]] = None,
         start_directory: t.Optional[str] = None,
         attach: bool = False,